U.S. patent number 7,602,529 [Application Number 10/935,460] was granted by the patent office on 2009-10-13 for method and system for controlling printer text/line art and halftone independently.
This patent grant is currently assigned to Eastman Kodak Company. Invention is credited to Thomas J. Foster, Gregory Rombola.
United States Patent |
7,602,529 |
Foster , et al. |
October 13, 2009 |
Method and system for controlling printer text/line art and
halftone independently
Abstract
A method of altering the appearance of an input digital image
when printed, the digital image comprised of an array of pixels and
wherein each pixel is assigned a digital value representing marking
information, and wherein the pixels comprise Text/Line pixels and
halftone pixels, the method comprising the steps of changing the
value of halftone pixels using a custom tone transfer function that
anticipates the effect of the reassigned pixel values on halftone
appearance; rasterizing the digital image; reassigning the digital
values of all pixels; and, printing the image.
Inventors: |
Foster; Thomas J. (Geneseo,
NY), Rombola; Gregory (Spencerport, NY) |
Assignee: |
Eastman Kodak Company
(Rochester, NY)
|
Family
ID: |
35197774 |
Appl.
No.: |
10/935,460 |
Filed: |
September 7, 2004 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20060050317 A1 |
Mar 9, 2006 |
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Current U.S.
Class: |
358/2.1; 358/1.9;
358/3.06; 382/100 |
Current CPC
Class: |
H04N
1/4092 (20130101); H04N 1/4072 (20130101) |
Current International
Class: |
H04N
1/40 (20060101); G06K 9/00 (20060101); H04N
1/405 (20060101); H04N 15/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 538 901 |
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Apr 1993 |
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EP |
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0 697 784 |
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Feb 1996 |
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EP |
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1 110 738 |
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Jun 2001 |
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EP |
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1 331 596 |
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Jul 2003 |
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EP |
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1 443 751 |
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Aug 2004 |
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EP |
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WO 01/89194 |
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Nov 2001 |
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WO |
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WO 02/10860 |
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Feb 2002 |
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WO |
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WO 02/14957 |
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Feb 2002 |
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WO |
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2005/084154 |
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Sep 2005 |
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WO |
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Primary Examiner: Haskins; Twyler L
Assistant Examiner: Reinier; Barbara D
Attorney, Agent or Firm: Zimmerli; William R. Watkins;
Peyton C.
Claims
The invention claimed is:
1. A method of printing a binary, multi-bit image comprising the
steps of: converting the image into a digital bitmap comprised of
an array of Text/line pixels and halftone pixels, wherein each
pixel is assigned a digital value representing marking information;
creating a custom tone transfer function created by comparing a
first output to a second output, embedded in the digital bitmap;
changing the value of halftone pixels using such custom tone
transfer function; rasterizing the digital bitmap; independently
reassigning the digital value of Text/Line pixels; and, printing
the image.
2. An apparatus for printing a binary, multi-bit image comprising:
a processor for converting the image into a digital bitmap
including an array of Text/line pixels and halftone pixels, wherein
each pixel is assigned a digital value representing marking
information, and a custom tone transfer function created by
comparing a first output to a second output, embedded in the
digital bitmap; changing the value of halftone pixels using said
custom tone transfer function embedded in the digital bitmap;
rasterizing the digital bitmap; independently reassigning the
digital value of Text/Line pixels; and, a print engine for printing
the image.
3. An apparatus for altering the appearance of an input digital
binary, multi-bit image when printed, the digital image comprised
of an array of pixels and wherein each pixel is assigned a digital
value representing marking information, and wherein the pixels
comprise Text/Line pixels and halftone pixels, the apparatus
comprising: a processor including a custom tone transfer function
created by comparing a first output to a second output, embedded in
a digital bitmap for changing the value of halftone pixels and
Text/Line pixels independently using such custom tone transfer
function; and, a print engine for printing the image.
Description
FIELD OF THE INVENTION
This invention is in the field of digital printing, and is more
specifically directed to image exposure control in
electrostatographic printers.
BACKGROUND OF THE INVENTION
Electrographic printing has become the prevalent technology for
modern computer-driven printing of text and images, on a wide
variety of hard copy media. This technology is also referred to as
electrographic marking, electrostatographic printing or marking,
and electrophotographic printing or marking. Conventional
electrographic printers are well suited for high resolution and
high speed printing, with resolutions of 600 dpi (dots per inch)
and higher becoming available even at modest prices. As will be
described below, at these resolutions, modern electrographic
printers and copiers are well-suited to be digitally controlled and
driven, and are thus highly compatible with computer graphics and
imaging. Controlling the appearance of printed images is an
important aspect of printers. An example of such control efforts is
described in U.S. Pat. No. 6,181,438, which is hereby incorporated
herein by reference.
Efforts regarding printers or printing systems have led to
continuing developments to improve their versatility practicality,
and efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a-1b are schematic diagrams of an electrographic marking or
reproduction system in accordance with the present invention.
FIG. 2 is a schematic block diagram for image rendering in
accordance with the present invention.
FIG. 3 is a flow chart for image rendering in accordance with the
present invention.
FIG. 4 is a schematic diagram of eight examples of directional
values assigned to pixels surrounding a pixel in question.
FIGS. 5a-5d are representations of a pixel grid having a toned
image provided thereon in accordance with the present
invention.
FIGS. 6a-6f are representations of a pixel grid having an image
provided thereon in accordance with the present invention.
FIG. 7a is a representation of a pixel grid having a one pixel wide
toned image provided thereon in accordance with the present
invention.
FIG. 7b is a representation of a pixel grid with edge pixel
designations for the toned image of FIG. 7a in accordance with the
present invention.
FIG. 7c is a representation of a pixel grid with direction values
for the toned image of FIG. 7a in accordance with the present
invention.
FIG. 7d is a representation of a pixel grid with background pixel,
edge pixel and one pixel wide line assignment values for the toned
image of FIG. 7a in accordance with the present invention.
FIG. 8a is a representation of a pixel grid having a two pixel wide
toned image provided thereon in accordance with the present
invention.
FIG. 8b is a representation of a pixel grid with edge pixel
assignments for the toned image of FIG. 8a in accordance with the
present invention.
FIG. 8c is a representation of a pixel grid with direction values
for the toned image of FIG. 8a in accordance with the present
invention.
FIG. 8d is a representation of a pixel grid with background pixel,
edge pixel and two pixel wide line assignment values for the toned
image of FIG. 8a in accordance with the present invention.
FIG. 9 is a schematic representation of an exemplary adjustment
interface for assigning new pixel values according to the present
invention.
FIG. 10 is a representation of a pixel grid with alternative pixel
assignments in accordance with the present invention for a letter
o.
FIG. 11 is an example of a tone reproduction curve for an
electrographic printer in accordance with the present
invention.
FIG. 12 is a copy of a series of printed halftone steps for three
different screen frequencies.
FIG. 13 is a graph illustrating percent lightness vs. percent black
pixels for each step for each screen frequency shown in FIG.
14.
FIG. 14 is a copy of a series of printed lines that are 1, 2, 3, 4,
and 8 pixels wide.
FIG. 15 is a graph illustrating linewidth vs. the number of pixels
counted across the line for an exemplary series of lines of FIG.
14.
FIG. 16 is a graph illustrating best fit lines extracted for
linewidth vs. the number of pixels derived by selecting a fixed IPV
and varying EPV, 2PV and 1PV for eight different cases.
FIG. 17 is a flow chart illustrating the steps taken to thin an
object by more than one pixel, in accordance with the present
invention.
FIG. 18a is a schematic diagram of pixel designations for a six by
six block of pixels in accordance with the present invention.
FIG. 18b is an eight bit digital number representative of the pixel
designations of FIG. 18a.
FIG. 18c is schematic diagram of a six by six block of pixels with
exemplary pixels marked.
FIG. 18d is a table of pixel designations for 256 possible marking
configurations for a six by six block of pixels in accordance with
the present invention.
FIGS. 19a-19e are representations of a pixel grid having toned
images provided thereon in accordance with the present
invention.
FIGS. 20a-20c are representations of a pixel grid having a toned
images provided thereon in accordance with the present
invention.
FIG. 21 shows a flow diagram depicting a method of matching the
outputs of two marking devices according to one presently preferred
embodiment of the invention.
FIG. 22 shows a graph of five UNDO tone transfer function curves
according to the present invention.
FIG. 23 shows a flow diagram illustrating a process of generating a
custom tone transfer function according to the present
invention.
DETAILED DESCRIPTION
Referring to FIG. 1, a printer machine 10 includes a moving
recording member such as a photoconductive belt 18 which is
entrained about a plurality of rollers or other supports 21a
through 21g, one or more of which is driven by a motor to advance
the belt. By way of example, roller 21a is illustrated as being
driven by motor 20. Motor 20 preferably advances the belt at a high
speed, such as 20 inches per second or higher, in the direction
indicated by arrow p, past a series of workstations of the printer
machine 10. Alternatively, belt 18 may be wrapped and secured about
only a single drum.
Printer machine 10 includes a controller or logic and control unit
(LCU) 24, preferably a digital computer or Microprocessor operating
according to a stored program for sequentially actuating the
workstations within printer machine 10, effecting overall control
of printer machine 10 and its various subsystems. LCU 24 also is
programmed to provide closed-loop control of printer machine 10 in
response to signals from various sensors and encoders. Aspects of
process control are described in U.S. Pat. No. 6,121,986
incorporated herein by this reference.
A primary charging station 28 in printer machine 10 sensitizes belt
18 by applying a uniform electrostatic corona charge, from
high-voltage charging wires at a predetermined primary voltage, to
a surface 18a of belt 18. The output of charging station 28 is
regulated by a programmable voltage controller 30, which is in turn
controlled by LCU 24 to adjust this primary voltage, for example by
controlling the electrical potential of a grid and thus controlling
movement of the corona charge. Other forms of chargers, including
brush or roller chargers, may also be used.
An exposure station 34 in printer machine 10 projects light from a
writer 34a to belt 18. This light selectively dissipates the
electrostatic charge on photoconductive belt 18 to form a latent
electrostatic image of the document to be copied or printed. Writer
34a is preferably constructed as an array of light emitting diodes
(LEDs), or alternatively as another light source such as a Laser or
spatial light modulator. Writer 34a exposes individual picture
elements (pixels) of belt 18 with light at a regulated intensity
and exposure, in the manner described below. The exposing light
discharges selected pixel locations of the photoconductor, so that
the pattern of localized voltages across the photoconductor
corresponds to the image to be printed. An image is a pattern of
physical light which may include characters, words, text, and other
features such as graphics, photos, etc. An image may be included in
a set of one or more images, such as in images of the pages of a
document. An image may be divided into segments, objects, or
structures each of which is itself an image. A segment, object or
structure of an image may be of any size up to and including the
whole image.
Image data to be printed is provided by an image data source 36,
which is a device that can provide digital data defining a version
of the image. Such types of devices are numerous and include
computer or microcontroller, computer workstation, scanner, digital
camera, etc. These image data sources are at front end and
generally include an application program that is used to create or
find an image to output. The application program sends the image to
the device driver, which serves as an interface between the client
and the marking device. The device driver then encodes the image in
a page description language ("PDL") and sends the encoded image to
the marking device. These data represent the location and intensity
of each pixel that is exposed by the printer. Signals from data
source 36, in combination with control signals from LCU 24 are
provided to a raster image processor (RIP) 37. The digital images
(including styled text) are converted by the RIP 37 from their form
in a page description language (PDL) language to a sequence of
serial instructions for the electrographic printer in a process
commonly known as "ripping" and which provides a ripped image to a
image storage and retrieval system known as a marking image
processor (MIP) 38.
In general, the major roles of the RIP 37 are to: receive job
information from the server; parse the header from the print job
and determine the printing and finishing requirements of the job;
analyze the PDL (page description language) to reflect any job or
page requirements that were not stated in the header; resolve any
conflicts between the requirements of the job and the marking
engine configuration (i.e., RIP time mismatch resolution); keep
accounting record and error logs and provide this information to
any subsystem, upon request; communicate image transfer
requirements to the marking engine; translate the data from PDL
(page description language) to raster for printing; and support
diagnostics communication between user applications. The RIP
accepts a print job in the form of a page description language
(PDL) such as postscript, PDF or PCL and converts it into raster,
or grid of lines or form that the marking engine can accept. The
PDL file received at the RIP describes the layout of the document
as it was created on the host computer used by the customer. This
conversion process is called rasterization. Another function of the
RIP is to perform or create custom transfer functions, such as an
UNDO transfer function 44. The UNDO transfer function will be
described in greater detail hereinafter. The RIP makes the decision
on how to process the document based on what PDL the document is
described in. It reaches this decision by looking at the beginning
data of the document. A job manager sends the job information to a
mss (marking subsystem services) via ethernet and the rest of the
document further into the RIP to get rasterized. For clarification,
the document header contains printer-specific information such as
whether to staple or duplex the job. Once the document has been
converted to raster by one of the interpreters, the raster data
goes to the MIP 38 via RTS (raster transfer services); this
transfers the data over a IDB (image data bus).
The MIP functionally replaces recirculating feeders on optical
copiers. This means that images are not mechanically rescanned
within jobs that require rescanning, but rather, images are
electronically retrieved from the mip to replace the rescan
process. The MIP accepts digital image input and stores it for a
limited time so it can be retrieved and printed to complete the job
as needed. The MIP consists of memory for storing digital image
input received from the rip. Once the images are in MIP memory,
they can be repeatedly read from memory and output to a RENDER or
rendering circuit 39. The amount of memory required to store a
given number of images can be reduced by compressing the images;
therefore, the images are compressed prior to MIP memory storage,
then decompressed while being read from MIP memory.
The output of the MIP is provided to an image RENDER circuit 39,
which performs or executes an algorithm which alters the image and
provides the altered image to the writer interface 32 (otherwise
known as a write head, print head, etc.) Which applies exposure
parameters to the exposure medium, such as a photoconductor 18. To
this end, RENDER circuit 39 may be described as an algorithm as it
may be performed either in hardware, software, or firmware.
After exposure, the portion of exposure medium belt 18 bearing the
latent charge images travels to a development station 35.
Development station 35 includes a magnetic brush in juxtaposition
to the belt 18. Magnetic brush development stations are well known
in the art, and are preferred in many applications; alternatively,
other known types of development stations or devices may be used.
Plural development stations 35 may be provided for developing
images in plural grey scales, colors, or from toners of different
physical characteristics. Full process color electrographic
printing is accomplished by utilizing this process for each of four
toner colors (e.g., black, cyan, magenta, yellow).
Upon the imaged portion of belt 18 reaching development station 35,
LCU 24 selectively activates development station 35 to apply toner
to belt 18 by moving backup roller 35a belt 18, into engagement
with or close proximity to the magnetic brush. Alternatively, the
magnetic brush may be moved toward belt 18 to selectively engage
belt 18. In either case, charged toner particles on the magnetic
brush are selectively attracted to the latent image patterns
present on belt 18, developing those image patterns. As the exposed
photoconductor passes the developing station, toner is attracted to
pixel locations of the photoconductor and as a result, a pattern of
toner corresponding to the image to be printed appears on the
photoconductor. As known in the art, conductor portions of
development station 35, such as conductive applicator cylinders,
are biased to act as electrodes. The electrodes are connected to a
variable supply voltage, which is regulated by programmable
controller 40 in response to LCU 24, by way of which the
development process is controlled.
Development station 35 may contain a two component developer mix
which comprises a dry mixture of toner and carrier particles.
Typically the carrier preferably comprises high coercivity (hard
magnetic) ferrite particles. As an example, the carrier particles
have a volume-weighted diameter of approximately 30.mu.. The dry
toner particles are substantially smaller, on the order of 6.mu. to
15.mu. in volume-weighted diameter. Development station 35 may
include an applicator having a rotatable magnetic core within a
shell, which also may be rotatably driven by a motor or other
suitable driving means. Relative rotation of the core and shell
moves the developer through a development zone in the presence of
an electrical field. In the course of development, the toner
selectively electrostatically adheres to photoconductive belt 18 to
develop the electrostatic images thereon and the carrier material
remains at development station 35. As toner is depleted from the
development station due to the development of the electrostatic
image, additional toner is periodically introduced by toner auger
42 into development station 35 to be mixed with the carrier
particles to maintain a uniform amount of development mixture. This
development mixture is controlled in accordance with various
development control processes. Single component developer stations,
as well as conventional liquid toner development stations, may also
be used.
A transfer station 46 in printing machine 10 moves a receiver sheet
s into engagement with photoconductive belt 18, in registration
with a developed image to transfer the developed image to receiver
sheet S. Receiver sheets S may be plain or coated paper, plastic,
or another medium capable of being handled by printer machine 10.
Typically, transfer station 46 includes a charging device for
electrostatically biasing movement of the toner particles from belt
18 to receiver sheet S. In this example, the biasing device is
roller 46b, which engages the back of sheet s and which is
connected to programmable voltage controller 46a that operates in a
constant current mode during transfer. Alternatively, an
intermediate member may have the image transferred to it and the
image may then be transferred to receiver sheet S. After transfer
of the toner image to receiver sheet s, sheet s is detacked from
belt 18 and transported to fuser station 49 where the image is
fixed onto sheet S, typically by the application of heat.
Alternatively, the image may be fixed to sheet s at the time of
transfer.
A cleaning station 48, such as a brush, blade, or web is also
located behind transfer station 46, and removes residual toner from
belt 18. A pre-clean charger (not shown) may be located before or
at cleaning station 48 to assist in this cleaning. After cleaning,
this portion of belt 18 is then ready for recharging and
re-exposure. Of course, other portions of belt 18 are
simultaneously located at the various workstations of printing
machine 10, so that the printing process is carried out in a
substantially continuous manner.
LCU 24 provides overall control of the apparatus and its various
subsystems as is well known. LCU 24 will typically include
temporary data storage memory, a central processing unit, timing
and cycle control unit, and stored program control. Data input and
output is performed sequentially through or under program control.
Input data can be applied through input signal buffers to an input
data processor, or through an interrupt signal processor, and
include input signals from various switches, sensors, and
analog-to-digital converters internal to printing machine 10, or
received from sources external to printing machine 10, such from as
a human user or a network control. The output data and control
signals from LCU 24 are applied directly or through storage latches
to suitable output drivers and in turn to the appropriate
subsystems within printing machine 10.
Process control strategies generally utilize various sensors to
provide real-time closed-loop control of the electrostatographic
process so that printing machine 10 generates "constant" image
quality output, from the user's perspective. Real-time process
control is necessary in electrographic printing, to account for
changes in the environmental ambient of the photographic printer,
and for changes in the operating conditions of the printer that
occur over time during operation (rest/run effects). An important
environmental condition parameter requiring process control is
relative humidity, because changes in relative humidity affect the
charge-to-mass ratio q/m of toner particles. The ratio q/m directly
determines the density of toner that adheres to the photoconductor
during development, and thus directly affects the density of the
resulting image. System changes that can occur over time include
changes due to aging of the printhead (exposure station), changes
in the concentration of magnetic carrier particles in the toner as
the toner is depleted through use, changes in the mechanical
position of primary charger elements, aging of the photoconductor,
variability in the manufacture of electrical components and of the
photoconductor, change in conditions as the printer warms up after
power-on, triboelectric charging of the toner, and other changes in
electrographic process conditions. Because of these effects and the
high resolution of modern electrographic printing, the process
control techniques have become quite complex.
Process control sensor may be a densitometer 76, which monitors
test patches that are exposed and developed in non-image areas of
photoconductive belt 18 under the control of LCU 24. Densitometer
76 measures the density of the test patches, which is compared to a
target density. Densitometer may include an infrared or visible
light led, which either shines through the belt or is reflected by
the belt onto a photodiode in densitometer 76. These toned test
patches are exposed to varying toner density levels, including full
density and various intermediate densities, so that the actual
density of toner in the patch can be compared with the desired
density of toner as indicated by the various control voltages and
signals. These densitometer measurements are used to control
primary charging voltage V.sub.o, maximum exposure light intensity
E.sub.o, and development station electrode bias V.sub.b. In
addition, the process control of a toner replenishment control
signal value or a toner concentration setpoint value to maintain
the charge-to-mass ratio q/m at a level that avoids dusting or
hollow character formation due to low toner charge, and also avoids
breakdown and transfer mottle due to high toner charge for improved
accuracy in the process control of printing machine 10. The toned
test patches are formed in the interframe area of belt 18 so that
the process control can be carried out in real time without
reducing the printed output throughput. Another sensor useful for
monitoring process parameters in printer machine 10 is electrometer
probe 50, mounted downstream of the corona charging station 28
relative to direction p of the movement of belt 18. An example of
an electrometer is described in U.S. Pat. No. 5,956,544
incorporated herein by this reference.
Other approaches to electrographic printing process control may be
utilized, such as those described in international publication
number WO 02/10860 a1, and international publication number WO
02/14957 A1, both commonly assigned herewith and incorporated
herein by this reference.
Raster image processing begins with a page description generated by
the computer application used to produce the desired image. The
raster image processor interprets this page description into a
display list of objects. This display list contains a descriptor
for each text and non-text object to be printed; in the case of
text, the descriptor specifies each text character, its font, and
its location on the page. For example, the contents of a word
processing document with styled text is translated by the RIP into
serial printer instructions that include, for the example of a
binary black printer, a bit for each pixel location indicating
whether that pixel is to be black or white. Binary print means an
image is converted to a digital array of pixels, each pixel having
a value assigned to it, and wherein the digital value of every
pixel is represented by only two possible numbers, either a one or
a zero. The digital image in such a case is known as a binary
image. Multi-bit images, alternatively, are represented by a
digital array of pixels, wherein the pixels have assigned values of
more than two number possibilities. The RIP renders the display
list into a "contone" (continuous tone) byte map for the page to be
printed. This contone byte map represents each pixel location on
the page to be printed by a density level (typically eight bits, or
one byte, for a byte map rendering) for each color to be printed.
Black text is generally represented by a full density value (255,
for an eight bit rendering) for each pixel within the character.
The byte map typically contains more information than can be used
by the printer. Finally, the RIP rasterizes the byte map into a bit
map for use by the printer. Half-tone densities are formed by the
application of a halftone "screen" to the byte map, especially in
the case of image objects to be printed. Pre-press adjustments can
include the selection of the particular halftone screens to be
applied, for example to adjust the contrast of the resulting
image.
Electrographic printers with gray scale printheads are also known,
as described in international publication number WO 01/89194 a2,
incorporated herein by this reference. The rendering algorithm
groups adjacent pixels into sets of adjacent cells, each cell
corresponding to a halftone dot of the image to be printed. The
gray tones are printed by increasing the level of exposure of each
pixel in the cell, by increasing the duration by way of which a
corresponding led in the printhead is kept on, and by "growing" the
exposure into adjacent pixels within the cell.
Ripping is printer-specific, in that the writing characteristics of
the printer to be used is taken into account in producing the
printer bit map. For example, the resolution of the printer both in
pixel size (dpi) and contrast resolution (bit depth at the contone
byte map) will determine the contone byte map. As noted above, the
contrast performance of the printer can be used in pre-press to
select the appropriate halftone screen. RIP rendering therefore
incorporates the attributes of the printer itself with the image
data to be printed.
The printer specificity in the RIP output may cause problems if the
RIP output is forwarded to a different electrographic printer. One
such problem is that the printed image will turn out to be either
darker or lighter than that which would be printed on the printer
for which the original RIP was performed. In some cases the
original image data is not available for re-processing by another
RIP in which tonal adjustments for the new printer may be made.
FIG. 2 illustrates a schematic block diagram of the function of
RENDER circuit 39. For exemplary purposes only, it is assumed that
binary image data is provided by the RIP on line 310 to converter
circuit 312 which, in this example, converts the data from binary
to multi-bit data, such as eight bit data. For example, the pixel
value may be converted from a 1 or 0 value, to a value ranging from
0 to 255 and provided on a line 314. For simplicity, it will be
assumed that the pixel being treated or the pixel in question (PIQ)
values on line 314 is either 0 or 255. The 8 bit PIQ value is
provided to an edge determination circuit 316 which applies a
standard 3.times.3 edge Laplacian kernel circuit to determine if
the PIQ is an edge pixel. The results (a) of this edge
determination is provided on a line 317 to mapping circuit 318 and
a pixel object width determination circuit 320. In other
terminology, circuit 316 flags whether the PIQ is edge pixel or
not. An edge is defined as a transition between background and
foreground. Edge pixels define the transition between background
and foreground pixels. Background pixels are defined as pixels
having relatively little or no marking information within. Print or
marking information is the digital value assigned to the pixel
which results in a certain amount of marking material, such as ink
or toner, to be deposited on a receiver, where the amount of
material has a functional relationship to the digital value. For
example, in the present embodiment, higher digital values may mean
higher amounts of toner being deposited, resulting in a visually
darker pixel. An inverse relationship could also be employed,
however. Foreground pixels are defined as pixels having some
printable or marking information within. Foreground pixels may be
either interior pixels, edge pixels, one line pixels, or two line
pixels. Interior pixels are foreground pixels that are not edge
pixels, one line pixels, or two line pixels.
The output on line 314 from converter circuit 312 is also provided
to a 3.times.3 directional look up table circuit 322. Circuit 322
assigns a direction value to each pixel. The directional assignment
is determined by the values of the eight pixels surrounding the
pixel. FIG. 4 illustrates an example of eight possible unique
directional assignments (N, NE, NW, S, SE, SW, E, W). The letter
designations indicate the direction of the adjoining pixels. One
way to interpret the letter designations is to consider where the
mass of adjoining black pixels are relative to the center pixel in
the 3.times.3. For example, the n assignment is that the direction
of adjoining pixels relative to the pixel in question is that they
lie to the north of it. Since there are 8 pixels surrounding the
pixel in question in a 3.times.3 region, there are 256 possible
pixel combinations. Each pixel combination yields one of the 8
possible directional values or a zero. A zero (or other designated
value) indicates that none of the directional values apply. FIG.
18d provides the complete 256-entry table. Note that each LUT entry
is assigned one of nine possible directional descriptive assignment
(eight examples of which are shown in FIG. 4). Other letters or
numerical designations may just as well have been assigned. The
output (D) of the directional LUT circuit 322 is provided on line
323 to character or object pixel width determination circuit
320.
Pixel width determination circuit 320 determines if the edge PIQ is
part of an object that is one or two pixels wide, and flags the
data with a tag (B) accordingly on a line 321. The tag can take on
one of three states or values. The PIQ can be part of a one pixel
wide object, a two pixel wide object, or neither. Any number of
algorithms can be utilized to perform this determination. The
present invention uses information obtained from the directional
LUT block 322, in which the detection circuit examines the
directional value of pixels surrounding the PIQ to identify pixels
that are part of a one or two pixel wide object, or neither. Refer
to FIGS. 7 and 8.
The following represents pseudo code for 1 pixel wide line pixel
value assignment decisions in accordance with the exemplary
algorithm for block 320 of FIG. 2: If pixel from a is an edge pixel
and pixel value from DIR LUT is 0, Then pixel is part of 1 pixel
wide line.
The following represents pseudo code for a 2 pixel wide line pixel
value assignment decisions in accordance with exemplary algorithm
for block 320 of FIG. 2: If pixel from a is an edge pixel, Then if
pixel from DIR LUT is a E and if adjacent pixel to the right is a W
Then pixel is part of a two pixel wide line Else if pixel from DIR
LUT is a SE and if pixel on next line and to the right is a NW Then
pixel is part of a two pixel wide line Else if pixel from DIR LUT
is a s and if pixel on next line and directly below is a N Then
pixel is part of a two pixel wide line Else if pixel from DIR LUT
is a SW and if pixel on next line and to left is a NE Then pixel is
part of a two pixel wide line Else if pixel from DIR LUT is a W and
if adjacent pixel to the left is a E Then pixel is part of a two
pixel wide line Else if pixel from DIR LUT is a NW and if pixel on
previous line and to left is a SE Then pixel is part of a two pixel
wide line Else if pixel from DIR LUT is a N and if pixel on
previous line and directly above is S Then pixel is part of a two
pixel wide line Else if pixel from DIR LUT is a NE and if pixel on
previous line and to right is a SW Then pixel is part of a two
pixel wide line Else pixel is an edge pixel
Mapping circuit 318 is provided information from multiple sources
and provides an output on a line 340 to the writer interface. The
inputs to mapping circuit are the edge detection pixel information
a on line 317, object width information B on a line 321, and
original image PIQ data C on line 314. In addition, assignment
values for interior pixels, edge pixels, one pixel wide lines, two
pixel wide lines and whether the algorithm is in a thinning or
thickening mode are provided to mapping circuit 318 on lines 330,
332, 334, 336 and 338. These assignment values are new values that
will be given to the PIQ, depending upon whether the PIQ is part of
a two pixel wide object (2PV), or if the PIQ is part of a one pixel
wide object (1PV), or if the PIQ is an edge pixel of an object more
than two pixels wide (EPV), and another value if the PIQ is an
interior (not background) pixel (IPV). Background pixels (white
area) are not changed by this particular algorithm, although
another might do so to achieve a desired effect.
The types of assignment parameters and the number of assignment
values may be determined in an unlimited number of ways. For
example, they may be provided by a user in response to a particular
effect the print operator wishes to obtain by programming through a
user interface, mechanical switches or other adjustments. The
assignment values may also be determined automatically by the
controller or LCU in response to printer operational parameters,
operator input or other input. The assignment values and parameters
may be combined to determine new assignment parameters. However
they may be determined, new pixel tone or exposure values will be
assigned to the PIQ post rip. One primary factor in new pixel tone
value assignment is the location of the PIQ in the image in
relation to surrounding pixels. Although the input to the RENDER
circuit 39 is explained as a binary input, the input may also be a
multi-bit input wherein new multi-bit PIQ exposure values will be
assigned for the input PIQ exposure values.
RENDER circuit 39 is an in line interface, or serial interface in
that it is provided between the RIP and the writer interface. Image
rendering can therefore be accomplished independent of the printer
or other printer components discussed hereinbefore, such as the RIP
or writer interface. It may be implemented with hardware (such as a
computer or processor board), software, or firmware as those terms
are known to those skilled in the art. The image rendering of the
present invention can also be accomplished utilizing data from the
other printer components, such as data typically utilized for
process control. In addition, image rendering may be set or
programmed by an operator or other external or remote source in
order to achieve a particular effect or effects in the printed
output. Implementing a rendering circuit in hardware just prior to
gray level writer allows for lower bandwidth requirements right up
to last stage before exposure. The writer may be any grey level
exposure system.
Referring to FIG. 3, a flowchart of a mapping function performed by
circuit 318 is provided. Data is provided by blocks 312, 316, 322
and on lines 330, 332, 334, 336 and 338. In a first step 210,
binary image data is received from the data source 36, preferably
after it has been ripped by the RIP 37. In a step 212, the mapping
function determines whether the pixel being treated or the pixel in
question (PIQ) is an edge pixel. Edge pixels of binary images may
be detected using any of a number of standard algorithms known in
the art (William k. Pratt, digital image processing, second
edition, John Wiley and sons, 1991, chapter 16). The edge can be
the white edge or black edge. The black edge is used for "thinning"
or lightening and the "white edge" is used for thickening or
darkening. To detect black edges, the binary image is converted to
8 bits (e.g. 0-->0 and 1-->255) and a standard 3.times.3 edge
Laplacian kernel is applied. Preferred embodiment uses the
following kernel:
##EQU00001##
The result of this operation is an image in which all image pixels
are 0 except for edge pixels which have a value of 255. To detect
white edges, the binary image is converted to 8 bits and inverted
(e.g. 0-->255 and 1-->0). White edges of text and other
features are detected when the image is to be darkened or lines and
halftones dots are to be made wider. The white pixel edges are then
replaced with a gray level to widen or extend the exposed region.
The amount of gray level added determines the degree to which the
image is darkened. The particular edge detection algorithm utilized
can be combinations and refinements of standard algorithms known in
the art. In a thinning case, changing the edge pixels of each
halftone cell to gray lightens the printed pictorial image. In a
thickening case, adding gray to the white edge pixels around the
halftone cell darkens the image.
If the PIQ is determined not to be an edge pixel, then in a step
214, the determination is made whether the PIQ value is zero or
something other than zero. If the PIQ value is zero, then the PIQ
value is assigned the background pixel value BPV, (which for
exemplary purposes in this case is zero) in a step 215, since it is
part of the background. If the PIQ value is not zero then it's
assumed it's an interior pixel (solid area pixel) and a new
interior pixel value (IPV) is assigned to it in a step 216.
If the PIQ in step 212 was determined to be an edge pixel, a step
217 determines whether the image rendering is in a thinning mode or
a widening mode. These modes will be discussed in more detail
hereinafter. If a thinning mode is desired, then a determination is
made in a step 218 as to whether the PIQ is an edge pixel of a line
or object that is one pixel wide. If yes, then the PIQ is assigned
a new one pixel wide value (1PV) in a step 220. If the answer to
step 218 is no, then a determination is made in a step 222 whether
the PIQ is part of a line or object that is two pixels wide. If yes
in step 222, then a two pixel wide value (2PV) is assigned to the
PIQ in a step 224. If no, then the edge pixel value (EPV) is
assigned to the PIQ in a step 226.
If the answer to step 217 is no, then the PIQ is assigned an edge
pixel value (EPV) in a step 226.
It is to be noted that the flowchart of FIG. 3 may be an algorithm
that is performed as part of the mapping circuit 318 or function as
illustrated in FIG. 2. Also, as can be seen in FIG. 2, binary pixel
data is provided by the RIP to the input of the image RENDER
circuit and multi-bit pixel data is output to the writer.
Variations of how the data is converted, and what values are
assigned to the different pixels are limitless and depend on what
alterations to printed images is desired by the user. Also, it can
be seen that the rendering algorithm begins with or is based on
detecting edges or edge pixels.
One implementation of this invention uses directional value
assigned by the LUT block to further classify an edge pixel by
direction. Edge pixels can be identified as pixels that occur at
specific orientations relative to the objects which they border.
This can be accomplished in the map block 318 of FIG. 2 by
providing directional information for the PIQ from line 323. When a
PIQ is determined to be an edge pixel, examining the directional
value assigned by block 322 for that pixel can further refine the
edge classification. In this way, unique values can be assigned to
edge pixels that are designated as one of the eight unique
directional values. In such an enhanced implementation, line 332
into block 318 would consist of eight unique assignment values, one
for each of the eight directional edge values. These can then take
on any combination of values including the same value. As an
example, all pixels with a N, NE and NW orientation may require
more aggressive thinning than pixels with other orientations. In
such an instance, all N, NE, NW edge pixels could be assigned a
grey level different from the remaining edge pixels. There may be
any number of applications or reasons to assign different values to
edge pixels based on orientation of surrounding pixels.
Referring to FIG. 4, a binary bitmap of eight different relational
configurations or objects in a 3.times.3 array of pixels are
defined as to where the PIQ is located with relation to the
surrounding object. In each array, the center pixel is considered
the PIQ. The eight possibilities are provided through a directional
look up table (DIR LUT) or directional LUT. Eight variable values
S, N, E, W, NE, SW, SE, NW are assigned the eight configurations.
It is to be noted that FIG. 4 illustrates only eight of 256
possible combinations of pixel patterns surrounding the PIQ. In the
present example though, other combinations result in one of the
eight relational assignments or zero, (zero indicates that none of
the directional assignments apply). In this manner, determination
of the orientation of the PIQ with respect to adjacent pixels can
be made.
As described hereinbefore, the RIP provides image data to a RENDER
circuit 39. The RIP 37 and RENDER circuit 39 can be dedicated
hardware, or a software routine such as a printer driver, or some
combination of both, for accomplishing this task.
The RENDER circuit 39 algorithm defines, classifies or identifies
each pixel as a particular kind of pixel and reassigns pixel values
as a function of their classification, where the different
classification reassignment values may be independent of each
other. For example, the algorithm may classify each pixel as either
a background pixel, interior pixel, edge pixel, one line pixel, or
two line pixel and reassign new values to these pixels according to
those classifications and independent of the each other. For
example, interior pixels may be reassigned new values while edge
pixel remained unchanged, or edge pixels may be reassigned new
values while leaving interior pixels unchanged, or edge pixel
values may be lowered with respect to interior pixel values, or
interior pixel values may be lowered with respect to edge pixel
values, etc. It can be seen there are unlimited variations to the
present rendering algorithm. Examples of the many pixel
classifications or assignments that may be assigned are defined
herein with the designations background pixel (BP), foreground
pixel (FP), interior pixel (IP), edge pixel (EP), one line pixel
(1W), two line pixel (2W), N, S, E, W, NE, NW, SE, SW, Y, Z,
etc.
Referring to FIGS. 6a-6f, wherein a character is represented in a
pixel grid. FIG. 6a is an illustration of a binary bitmap of a
character. It can be seen that the pixels are either all black
(filled with solid area density of maximum toner Dmax) or have no
toner and have area toner density of zero.
FIG. 6b illustrates the toned character of 6a after assigning a
lower pixel value to both interior pixels and edge pixels. In other
words, IPV and EPV were reassigned from Dmax in FIG. 6a to Dx,
where Dx is lower than Dmax.
FIG. 6c illustrates the edge pixels of the character when the
character is undergoing thinning.
FIG. 6d illustrates assignment of new EPV and IPV values for the
edge and interior pixels of the character after thinning has
occurred.
FIG. 6e illustrates the edge pixels of the character when the
character is undergoing thickening.
FIG. 6f illustrates assignment of new EPV and IPV values for the
edge and interior pixels of the character after thickening has
occurred.
It can be seen from these Figures that after operation of the
algorithm, the edge pixels and interior pixels may be assigned grey
levels (or marking values) independently. Once edge pixels are
detected, the remaining pixels consist of either "background"
pixels (white unprinted area) or "interior" pixels (foreground less
edge pixels). Interior pixels can be distinguished from background
pixels in that if a pixel is not an edge pixel (from above) and if
in the original image data the pixel is a 0 (no marking) then the
pixel is a background pixel. On the other hand, if the pixel is not
a edge pixel (from above) and if in the original image data the
pixel is a 1 (marking) then the pixel is an interior pixel. With
the RENDER circuit, the exposure level of interior pixels can be
changed. Second and subsequent layers of edge pixels can be
detected by simply performing the edge detection algorithm on the
interior pixels which remain after the edge pixels are removed.
Interior pixels would then refer to pixels remaining after all
layers of edge pixels have been removed. A flow chart for this type
of iteration is illustrated in FIG. 17.
The steps taken in FIG. 17 begin with step 610 of receiving image
bitmap data. In a step 612, the edge pixels are identified and
assigned a new value EPV1 in a step 614 and thereafter sent to the
writer. The EDGE 1 pixels identified in step 612 are also assigned
a value of zero in a step 616, thereby creating a new "virtual"
edge 2. The EDGE 2 pixels are identified in a step 618 and
reassigned a new pixel value EPV2 in a step 620 and thereafter sent
to the writer. The EDGE 2 pixels identified in step 618 are also
assigned a value of zero in a step 622, thereby creating a new
"virtual" edge 3. The EDGE 3 pixels are identified in a step 624
and reassigned a new pixel value EPV3 in a step 626 and thereafter
sent to the writer. This process can be iterated many times over so
that edge n-1 pixels are assigned a value of zero in a step 628,
thereby creating a new "virtual" edge n. The EDGE N pixels are
identified in a step 630 and reassigned a new pixel value EPVN in a
step 632 and thereafter sent to the writer.
A process similar to that described above process may be utilized
to thicken or expand the size of an object edges by simply
assigning a value higher than zero, such as one or Dmax in steps
616, 622, 628, etc. In order to create a new edge, real or
virtual.
FIGS. 5a-5d illustrate different alterations that may be
accomplished using an iterative edge detection.
FIG. 5a illustrates the original object. FIG. 5b illustrates four
layers of edge pixels identified by iteratively thinning. The
outermost layer representing the edge pixels of the original
object. FIG. 5c illustrates three layers of edge pixels when
iteratively thickening. The innermost layer represents the edge
pixels just outside of the original object. FIG. 5d illustrates the
combined layers of FIGS. 5b and 5c.
Referring to FIGS. 7a-7d in conjunction with FIGS. 2 and 3, wherein
a single pixel width character or line is represented in a pixel
grid. FIG. 7a is an illustration of a binary bitmap of a one pixel
wide character. It can be seen that the pixels are either all black
(filled with solid area density of maximum toner Dmax) or have no
toner and have area toner density of zero. FIG. 7b illustrates
assignment of eight bit values for the binary values of FIG. 7a
after determination of the edge pixels according to a laplacian
kernel. FIG. 7c illustrates assignment of direction values for the
pixels surrounding character pixels after application of the
directional assignment algorithm of block 322 of FIG. 2 and LUT of
FIG. 18d. FIG. 7d illustrates the assignment of background pixel,
edge pixel and direction values for the pixel grid in accordance
with the pseudo code algorithm described hereinbefore.
Referring to FIGS. 8a-8d in conjunction with FIGS. 2 and 3, wherein
a two pixel width character or line is represented in a pixel grid.
FIG. 8a is an illustration of a binary bitmap of a two pixel wide
character. It can be seen that the pixels are either all black
(filled with solid area density of maximum toner Dmax) or have no
toner and have area toner density of zero. FIG. 8b illustrates
assignment of eight bit values for the binary values of FIG. 8a
after determination of the edge pixels according to a Laplacian
kernel. FIG. 8c illustrates assignment of direction values for the
pixels surrounding character pixels after application of the
directional assignment algorithm of block 322 of FIG. 2 and LUT of
FIG. 18d. FIG. 8d illustrates the assignment background pixel, edge
pixel and of direction values for the pixel grid in accordance with
the pseudo code algorithm described hereinbefore.
As described, in order to preserve fine lines (avoid loss of
information), one and two pixel wide lines are each detected when
"thinning". All pixels that comprise a one or two pixel wide line
are categorized as edge pixels after the Laplacian operation. It is
to be appreciated that many methods known in the art can be used to
identify 1 and 2 pixel wide lines. As described herein, to
distinguish 1 and 2 pixel wide lines from other edge pixels, the
original image which has been converted to 8 bits is operated upon
by a 3.times.3 direction look up table (DIR LUT). The resulting
output contains information identifying the edge gradient of all
edges. Using information from the original image, the output of
this operation along with edge pixel data from the image created by
the Laplacian operation is used to identify pixels which are part
of a one pixel wide line from pixels which are part of a two pixel
wide line. Since one pixel wide lines can be detected and
distinguished from 2 pixel wide lines, each type of line can have a
unique gray level assigned to it which in turn can be different
from other edge pixels.
Note that if iterative thinning or thickening is applied, there may
exist first layer edge pixel values, second layer pixel values,
etc. The gray level range for interior and edge pixels is 0 (no
exposure) to 255 (maximum exposure). When thinning, one and two
pixel wide lines have a range from some minimum exposure (not 0) to
the maximum exposure. This is so that these lines will appear on
the print. However, the present invention does not preclude setting
gray level on these in order to intentionally erase fine lines.
FIG. 9 illustrates an example of an interface for an operator to
adjust the pixel density assignment values. Other inputs can be
utilized. As discussed previously, an operator can adjust these
parameters in different ways to achieve a desired print result. For
exemplary purposes only, there is shown adjustments for the values
of interior pixel, edge pixel, one pixel wide, two pixel wide,
toner consumption, character linewidth, shadow, asymetry and
exposure modulation (lightness/darkness). The adjustments can be
made utilizing a user interface or mechanical switch connected to
the printer, the particular kind and style of interface being
variable. Providing a user with an interface allows that user to
make many adjustments to the image so as to achieve a particular
print output without having to rerip the image. As discussed
herein, different printers provide different print characteristics.
The user interface provides a means to adjust one printer to mimic
or appear like another printer on the fly, so to speak. That is,
adjustments can be made while the printer is operating so that
print output may be analyzed quickly and iteratively with little
inconvenience. Not all of the adjustments in FIG. 9 would be
located in the same interface, and other adjustments not
specifically shown therein are contemplated.
The present RENDER circuit may be used in any type of digital
printing system, such as electrostatographic, electrophotographic,
inkjet, laser jet, etc. Of any size or capacity in which pixel
exposure adjustment value is selected prior to printing. The
printer processes a bit map of the image to be printed and
identifies edge pixels first and then identifies other types of
pixels in that image. The exposure level for these pixels is then
set by the printer according to new pixel exposure adjustment
values according to density adjustments performed by the printer.
Many printed image and object characteristics, parameters and
utilities may be affected by this method. For instance, a pattern
may be provided to interior pixels. This would be applied in the
mapping section where the interior pixel value is assigned. A
benefit to the present algorithm is that changes may take effect
immediately because process control controls to the same
density.
When combining output from different printers to create one
document, it is sometimes desirable to have the look and feel of
the printers to be as similar as possible. Also, bitmaps of images
ripped on one printer are sometimes printed on a printer with
different characteristics than the original printer for which they
were ripped. The present invention provides a method to obtain this
result without reripping images and without adjusting other machine
setup parameters (e.g. Electrostatographic process setpoints).
Appearance aspects which may be adjusted include but are not
limited to text, line widths and pictorial tone scale. Feel aspect
include but are not limited to toner stacking (tactile feel of
toner stack). Image adjustments made utilizing the RENDER circuit
described herein take immediate effect on print output and
therefore avoids any time delays normally associated with closed
loop control system adjustment to electrostatographic process
setpoints.
Sometimes users are willing to tradeoff image quality to attain
higher toner yield per printed page. Another aspect of the RENDER
circuit is to provide the user with a "knob" or adjustment to
adjust toner consumption at various levels of image quality, as
shown in FIG. 9. A user is provided the ability to lower certain
pixel values, like interior and edge pixels, thereby lowering the
amount of toner being deposited in the affected pixels and thereby
lowering overall toner consumption. A user can adjust the printed
image in this manner so as to minimize toner consumption while
maintaining acceptable image quality without having to rerip the
image.
To this end, it can be seen that the RENDER circuit accounts for
all pixels of an image to be printed, and determines toner levels
for each pixel. With this being the case, the printer may track or
monitor total toner consumption of the printer accurately by adding
or calculating the toner deposited for each line, character, and
image processed and printed. By counting the number of edge (those
having at least one adjacent pixel non toned) and interior (those
having all adjacent pixels toned) and applying different conversion
factors (toner usage per pixel to each), a prediction of toner
usage can be achieved. Toner consumption by line, page, job or
multiple jobs can be accomplished. This estimate has customer
applications as well as potential uses in toner replenishment/toner
concentration control in the printer itself. The conversion factors
applied can also be dependent on the density targets used in
printers that have variable density control allowing the customer
to select the best cost/quality point for each job. As an example,
6% coverage documents made up of text and made up of 1 inch solid
squares have been shown to consume between 0.0397 and 0.0294 grams
of toner per sheet respectively. This difference of 33% occurs even
though the total number of black pixels for the two documents
differs by less than 0.5% analyzing these two images for edge and
interior pixels indicates that edge pixels consume 1.3 times the
toner that interior pixels do. Accounting for the edge and interior
pixels separately clearly yields improved estimates for toner
consumption than estimates using only pixel counts.
As mentioned hereinbefore, the process of electrostatography or
electrography involves forming an electrostatic charge image on a
dielectric surface, typically the surface of a photoconductive
recording element that is being drawn or otherwise conveyed through
a developing station or toning zone. The image is developed by
bringing a two-component developer into contact with the
electrostatic image and/or the dielectric surface upon which the
image is disposed. The developer includes a mixture of pigmented
resinous particles generally referred to as toner and
magnetically-attractable particles generally referred to as
carrier. The nonmagnetic toner particles impinge upon the carrier
particles and thereby acquire a triboelectric charge that is
opposite the charge of the electrostatic image. The developer and
the electrostatic image are brought into contact with each other in
the toning zone, wherein the toner particles are stripped from the
carrier particles and attracted to the image by the relatively
strong electrostatic force thereof. Thus, the toner particles are
deposited on the image. The magnetic carrier particles are drawn to
the toning shell by the rotating magnets therein. This magnetic
force generally does not affect the nonmagnetic toner
particles.
However, within the toning zone the toner particles are affected by
forces other than the electrostatic force attracting the toner to
the image and which may degrade image quality. These forces
include, for example, repulsion of toner from the portion of the
dielectric surface or photoconductive element that corresponds to
the background area of the image, electrical attraction of the
toner particles to the carrier particles, repulsion of toner
particles from other toner particles, and electrical attraction to
or repulsion from the toning shell depending on the polarity of the
film voltage in the developer nip area. There are certain methods
of compensating for and/or balancing the effect of these other
forces on the nonmagnetic toner particles to prevent any
significant adverse effect on image quality. However, the forces on
toner particles having magnetic content are very different from the
forces on nonmagnetic toner.
In addition to the electrical forces acting on nonmagnetic toner as
described above, toner having magnetic content is subjected to
magnetic forces, such as, for example, magnetic attraction of the
toner particles to the carrier particles, to other toner particles,
and to the rotating core magnet. All of these magnetic forces are
generally in a direction away from the film or electrostatic image
carrier. The only force acting to draw the toner onto the
electrostatic image carried by the film or dielectric carrier is
the electric force. Thus, the magnetic forces tend to counteract
the electric attraction of toner particles to the image. The
strength of the electric force relative to the magnetic forces
becomes stronger as the distance between the image and the core
magnet increases. Therefore, the toner tends to be deposited on the
trailing edge of the film or dielectric carrier. The result is an
image having solids with heavy toning on the trailing edge of the
image, and cross track lines (i.e., lines perpendicular to the
direction of travel of the dielectric support member or film) that
are wider than the corresponding in track lines (i.e., lines that
are parallel to the direction of travel of the dielectric support
member or film).
This "fringe" field effect (the condition wherein fringe
electromagnetic fields around the edges of lines on the
photoconductor result in toner build up at edges of lines on the
printed material) can be a problem for some printers. The RENDER
circuit described herein provides a method to reduce the toner
build up on the edges by adjusting the IPV, EPV, 1PV or 2PV
parameters accordingly to reduce or counteract these effects. For
example, FIGS. 5a-5d illustrate a character having different
exposure values assigned to different layers which may be utilized
to minimize the fringe field effect on image quality.
As described hereinbefore, Dmax control uses the signal from a
transmission densitometer circuit reading a Dmax patch to adjust V0
and/or E0 electrophotographic parameters concurrently to maintain
solid area density. In addition to Dmax, a shadow detail patch may
be written using approximately 70-90% pixel pattern or at 70-90% of
Dmax exposure in a flat field pattern at the selected edge and
interior pixel exposure values determined by the RENDER circuit
during tuning prior to the run. Based on the densitometer signal
generated by this patch, the edge and interior pixel exposure
values may be adjusted to maintain the desired shadow detail
density (or large line character width) by adjusting or reassigning
pixel values. In addition, a highlight detail patch may be written
using approximately 5-20% of Dmax exposure black pixels in a flat
field pattern using the selected edge, interior, and small feature
pixel exposure values determined by the RENDER circuit during
tuning prior to the run. Based on the densitometer signal generated
by this patch, the small feature pixel values may be adjusted to
maintain the desired highlight detail density (or fine line
character linewidth) by reassigning one or more of EPV, 1PV and
2PV.
As described herein, it is possible using the RENDER circuit to
apply reduced exposure at all edges of characters, but this may
lead to too large a reduction in line width since the minimum
adjustment is applied to two pixels. This is especially true of
characters printed in a small font size. To achieve less linewidth
reduction, half of character edges may be reduced (top and left
edges only for example). This may lead, however, to an apparent
shift of the center of the characters locations and this may be
undesirable for a particular application (for instance with kerned
fonts and small font size characters). To achieve linewidth
reductions less than those achieved with all edge pixel exposure
reductions, and avoid apparent center shifts of small font size
characters caused by top/left or bottom/right edge exposure
reductions, the RENDER circuit may apply an alternative algorithm
and assign pixel values such that closed characters (those having
enclosed spaces such as O, D, B, etc.) Have reduced exposure only
for the interior or exterior edges of enclosed areas. For example,
FIG. 10 illustrates a letter "O", (which is a closed character),
having interior edges and exterior edges with different exposure
values assigned to them. This helps to maintain the center location
of character without achieving excessive linewidth reduction.
Remaining straight portions of the characters may have only one
edge exposure reduced. A similar algorithm may be applied to
characters having partially enclosed spaces (such as V, C, M, N
etc.) Whereby only the interior or exterior edge is exposure
modified. Characters with multiple partially enclosed spaces (such
as T, Y, W, M, etc.) Would require a larger set of rules to avoid
modifying both edges of any strokes, but it should be possible to
generate a consistent set of rules capable of avoiding such
conflicts.
Desired edge exposure reductions may utilize a two dimensional
operator of sufficient size to completely enclose the largest size
character to which it will be applied. If an area is identified in
the operator field of view as a separate object, it may then
operate on the object in accordance with the rendering algorithm
described herein to reduce apparent linewidth while minimizing the
apparent center shifting of characters.
As the interior pixel (solid area density) exposures drop below
certain levels, electrophotographic process nonuniformities become
apparent in the solid area imaging. Assigning a pattern of
different exposure values for interior pixels (multiple IPVs rather
than using a single exposure for all interior pixels) reduces the
visibility of EP process non-uniformity. The particular pattern
used is analogous to a halftoning pattern for binary imaging,
except the modulation is between different non-white exposure
levels. The pattern of differing density pixels tends to obscure
streaks and bands that become visible in flat fields of same level
exposure pixels and minimizes the visibility of non-uniform
density. The nonuniformities can be identified or measured in a
number of ways, examples of which are visually inspecting the
printed output or utilizing a density patch and measuring density
thereof. The pattern can be of any size with any number of
different exposure values such that it creates the desired average
interior pixel density when printed to reduce print
nonuniformities.
In this regard, the present invention is useful when printing
magnetic toner or ink. Magnetic ink character recognition (MICR)
technologies have been used for many years for the automated
reading and sorting of checks and negotiable payment instruments,
as well as for other documents in need of high speed reading and
sorting. As well known in the art, MICR documents are printed with
characters in a special font (e.g., the e13-b MICR font in the
united states, and the cmc-7 MICR standard in some other
countries). Typically, MICR characters are used to indicate the
payor financial institution, payor account number, and instrument
number, on the payment instrument. In addition to the special font,
MICR characters are printed with special inks or toners that
include magnetizable substances, such as iron oxide, that are
magnetized for facilitating an automatic reading process by a
reading instrument which is sensitive to the magnetic fields
surrounding the printed MICR characters. The magnetized MICR
characters present a magnetic signal of adequate readable strength
to the reading and sorting equipment, to facilitate automated
routing and clearing functions in the presentation and payment of
these instruments.
The relatively heavy loading of iron oxide in conventional MICR
toner for electrographic MICR printing has been observed to
adversely affect the image quality of the printed characters,
however. It is difficult to achieve and maintain an adequate
dispersion of the heavy iron oxide particles in the toner resin. In
addition, the toning and fusing efficiencies of MICR toners are
poorer than normal (i.e., non-MICR) toners, because of the magnetic
loadings present in the MICR toner. Accordingly, the image quality
provided by MICR toner may be poorer than those formed by normal
toner, unless the printing machine makes adjustments to compensate.
The present RENDER circuit provides a way to adjust MICR toner
density in parts of characters so as to minimize the printing
nonuniformities resultant therefrom. By varying pixel toner density
values as a function of pixel character location as illustrated in
the exemplary drawings herein, the concentration of magnetic toner
particles may be adjusted to improve the readability of the printed
characters by reading instrumentation.
FIG. 11 illustrates an example of a typical tone reproduction
curve, also referred to in the art as a "gamma" curve, illustrating
the typical performance of conventional printers in reproducing
tone density, in this example for gray scale printing. In this
plot, the horizontal axis corresponds to input intensity between
white (no intensity) and black (full intensity); the vertical axis
corresponds to the corresponding printer output density, on the
hard copy medium, between d.sub.0 (no density) and Dmax (full
density). Ideally, the transfer function from input intensity to
output density would be a 45.degree. line, shown as ideal plot I in
FIG. 11, along which the output density exactly matches the input
intensity.
Printer performance follows a non-linear "S-shaped" tone
reproduction curve, for example as shown by actual plot a in FIG.
11, often referred to as the "gamma" curve. Along this tone
reproduction curve, output density is generally less than that
specified by low input intensity values (i.e., below the ideal I);
this portion of the tone reproduction curve is referred to as the
"toe", shown by region t in FIG. 11. The output densities in the
"toe" region are also referred to as "highlight" densities. At the
other extreme, for high input intensity values, output density is
generally higher than that specified by the input (i.e., above the
ideal I). These output densities in the "shoulder" region of the
tone reproduction curve, for example in region S of plot a in FIG.
11, are also referred to as "shadow" densities. For both the
highlight and shadow densities, the inaccuracy in tone reproduction
is generally manifest by inaccuracies in the printed contrast; the
underdensity in highlight regions shows up as washed out regions of
the image, while the overdensity in shadow regions shows up by the
absence of bright features (loss of detail in dark regions). In the
"midtone" region of the tone reproduction curve, shown by region MT
of plot a in FIG. 11, the error between output density and input
intensity is relatively small, so that midtones produced by the
printer closely match the input signal.
In many cases, the raster image processor (rip) described above, by
way of which a page description is converted into a bit map output
for printing by a specific printer of the electrographic or other
type, applies gamma correction in this processing. This gamma
correction compensates for the non-ideal density output of the
printer, in effect applying a transfer function that is the
opposite of the tone reproduction curve for the printer (e.g., plot
a of FIG. 11). This correction will generally be implemented by
increasing the density output for lower input intensity values, and
decreasing the density output for higher input intensity values. To
at least a first approximation, the correction amounts to the
selection of a gamma value, which is a compensating factor
corresponding to the degree of curvature of the actual tone
reproduction curve a from the ideal I. As noted above, the actual
correction may be carried out by selection of the appropriate
halftone screens using higher density halftone screens for
highlight densities, and lower density halftone screens for
shoulder densities.
According to conventional approaches, the selection of the
appropriate halftone screens for a given printer or printer type
requires a trial and error process. The correct Dmax output density
level must first be correlated to full density input. Once Dmax is
set, then a representative image is processed using a trial set of
corrections for highlight and shadow densities; after analysis of
the output image, the corrections may be adjusted and the image
processed again. Upon convergence to the desired output, additional
images may be adjusted using the corrections (e.g., the selected
set of halftone screens) determined in the trial and error process,
and printing can commence. To the extent that the iterative setting
of shoulder and toe corrections must be performed for a given
printer, or on specific images, this procedure is time consuming
and costly.
Because of printer specificity in the RIP process, RIP output for
one printer or printer type cannot be forwarded to a different
electrographic printer without risking that the printed image will
have incorrect gamma correction for the images. In other words, the
gamma correction in the RIP output based on the printer for which
the original RIP was performed will likely not correspond to the
tone reproduction curve of a different printer.
As discussed above, U.S. Pat. No. 6,121,986 provides a solid area
density control system, in which the optical density of maximum
density patches, and of less than maximum density patches, is
controlled in response to the measured performance of the
electrographic printer. This solid area density control adjusts the
output density Dmax during setup and operation of the printer, and
also can control the output density at different less-than-maximum
levels. However, this conventional solid area density control only
controls the solid area output density value Dmax, and cannot
separately control highlight and shadow densities. In other words,
an increase in solid area output density Dmax compensates for the
underdensity of highlights, but overcompensates for shadows.
Conversely, a decrease in Dmax compensates for the overdensity of
shadows, but undercompensates for highlights. While solid area
control approaches stabilize the optical density of the exposed
areas, they don't necessarily introduce variations into character
linewidths of text (and analogously into the linewidths of small
isolated image features). Linewidth variations are due in part to
fringe field effects. As known in the art, the amount of toner
applied to a pixel on the photoconductor of an electrographic
printer depends upon the difference between the exposure voltage
(as applied by the led or laser to the photoconductor) and the bias
voltage at the toning station; changes in either of these voltages
will change the amount of toner received by the pixel. Fringe
effects occur because the electric field at the edge of an exposed
patch (i.e., those edges of exposed pixels that are adjacent to
unexposed pixels) is much greater than the field at the center of
the exposed region. It has been observed that the difference in
field magnitude between the edge and the center may be as high as
3.times. to 5.times.. As a result, toner tends to pile up at the
edge of an exposed patch of pixels, and at the edge of single
exposed pixels surrounded by unexposed pixels. In the case of
single pixels, this piling effect can result in single pixel sizes
of on the order of 90.mu. in 600 dpi printers that have a
theoretical pixel pitch of 42.mu.. Again, these fringe effects
affect both gray scale images and also full-black text and make it
difficult to adjust image quality to the extent necessary to
compensate for differences in characteristics between an
electrographic printer for which the image was originally ripped,
and a different electrographic printer upon which the image is to
be printed. These fringe effects are reduced utilizing the RENDER
circuit of the present invention by reassigning edge pixels to have
lower exposure values (EPV) at the edge of an exposed patch of
pixels, and at the edge of single exposed pixels surrounded by
unexposed pixels.
Many printers cannot produce a continuous spectrum of tone levels
within an image. Instead, modulated patterns are used to simulate
different tone levels. One common form of modulation is the use of
halftones. For instance, many typical black and white printers use
halftones to produce a variety of gray levels, such as 30% white or
60% white. The use of halftones involves dividing the rasterized
image into halftone cells, each of which contains a fixed number of
pixels. A continuous tone spectrum thus may be approximated by
turning on a certain percentage of the pixels in each halftone
cell. Theoretically, a 50% gray level may be achieved by turning on
50% of the pixels in a halftone cell on a black and white printer.
Digitized halftone images processed at different effective screen
frequencies (the number of lines per inch or lpi) often have
different contrast (appearances) because of differing dot gains
depending on the ratio of edge and interior pixels as the area
coverage changes. FIG. 12 illustrates seventeen halftone steps (the
percentage of white in each step) for three different screen
frequencies, 106 lpi, 85 lpi and 71 lpi. The relationship of
percent lightness to percent black pixels for each step for each
screen frequency is shown in FIG. 13. It can be seen that the three
curves at standard exposure are different, thereby illustrating
different halftone images for different screen frequencies. FIG. 14
illustrates a series of lines that are 1, 2, 3, 4, and 8 pixels
wide, respectively. FIG. 15 illustrates a graph of linewidth vs.
the number of pixels counted across the line, where white spaces
are assigned negative numbers for a particular set of lines with
the same linewidth (for example 8 pixel wide lines). A best fit
line 500 can be drawn through the data points collected. FIG. 16
illustrates a series of best fit lines extracted for linewidth vs.
the number of pixels derived by selecting a fixed IPV and varying
EPV, 2PV and 1PV for eight different cases. It can be seen that
there are eight different best fit lines. It can also be seen that
there is one particular best fit line that passes through the zero
intercept. The EPV, 2PV and 1PV values for the zero intercept line
was noted and a series of lines similar to those shown in FIG. 14
were printed at screen frequencies of 106 lpi, 85 lpi and 71 lpi.
The relationship of percent lightness to percent black pixels for
the three screen frequencies were plotted and are shown in FIG. 13,
wherein the resulting curves identified as the zero intercept group
curves. It can be seen that using the EPV, 2PV and 1PV values for
the zero intercept line results in digitized halftone images that
are the same for differing screen frequencies. By using EPV, 2PV
and 1PV exposures that are different from IPV exposure, it is
possible to achieve linear behavior between character linewidth and
the number of pixels printed that has an intercept of zero. Because
the IPV exposure hasn't changed, it is possible to retain good
solid area fill by overlapping interior pixels. Because the
relationship between pixel width and measured width has a zero
intercept, image density for halftone patterns is not dependent on
the ratio of edge and interior pixels, which means that is it also
independent of screen frequency. Using a user interface, the user
is therefore able to adjust the solid area maximum density (IPV)
and then select edge pixel exposures (EPV, 2PV, 1PV) to achieve a
zero intercept of the character linewidth vs. number of pixels
curve to minimize screen frequency sensitivity. To this end,
sensitivity to screens having different dot shapes (e.g. round,
elliptical, diamond, etc.) May be minimized also.
Referring now to FIG. 18a, a 3.times.3 pixel array is illustrated.
In the array, the center pixel is the PIQ. There are eight pixels
surrounding the PIQ. These pixels have been assigned designations
b0 through b7. An eight bit binary number can be created by
associating a zero with an unmarked pixel and a 1 with a marked
pixel as shown in FIG. 18b. For instance, FIG. 18c shows an
exemplary pixel pattern adjacent to the PIQ. Assuming a marked
pixel has a value of 1 and a blank pixel has a value of zero, the
pattern in FIG. 18c (two marked pixels at bit positions B0 and B3)
would correlate to a binary number of 00001001 (9 in decimal)
according to the pixel designations defined in FIGS. 18a and
18b.
From FIG. 18b it can be derived that there are 256 different binary
numbers associated with the eight binary bits (e.g. 00000001,
00000010, 00000011, 00000100, etc.) Representing the eight pixels
surrounding the PIQ. A look up table (LUT) for the 256 entries may
be created.
FIG. 18d provides a 256 entry LUT, wherein each entry represents a
directional assignment (0, SE, S, SW, E, W, NE, N, NW) for the
pixels in the configuration defined by the eight bit binary number.
For example, in FIG. 18d, the entry in the first column of the
first row represents the binary number 00000000. The entry to the
right of 00000000 represents pixel configuration 00000001 wherein
only one pixel (labeled B0 in FIG. 18a) is marked. That pixel would
be given the directional assignment se. The entry to the right of
that is 00000010 (wherein only the pixel labeled B1 in FIG. 18a)
would be marked in accordance with the directional assignment s.
The table entry directly below the 00000000 entry would be
represented by the binary number 00010000 (16 in decimal), and
wherein the PIQ would be given a directional assignment of W. The
table entries shown in FIG. 18d for each of the 256 possible
configurations therefore represent one of nine directional
assignments for each pixel in that particular configuration.
FIG. 19a is an illustration of a binary bitmap of a character in a
pixel grid having two portions 710, 712, with differing pixel toner
densities or other values. Portion 712 could, for example, be
intended to be provide a shadow effect to portion 710. If portions
710 and 712 were intended to be contiguous, then FIG. 19a
illustrates a case where the portions are misaligned (a case that
is highly probable) wherein a one pixel wide blank line 714 lies
between them. The misalignment may be a result of any number of
registration problems originating from mechanical, electrical, or
software discrepancies. Either the outer edge of portion 710, or
the shadow portion 712 could be identified post RIP utilizing the
directional and one pixel wide line recognition capabilities of the
render algorithm described herein. For example, the pixels creating
structure 712 would be identified as either N, NE, or E pixels.
FIG. 19b illustrates the one pixel wide outer edge 716 of portion
710 that is nearest portion 712 identified using the edge and
directional capabilities of the present render algorithm. For
example, the pixels of edge 716 would be identified as either N,
NE, or E pixels.
Referring to FIG. 19c, once identified, the edge could be thickened
into a two pixel wide structure 716 utilizing the present
algorithm.
Referring to FIG. 19d illustrates a new portion 710' and portion
712 printed together. It can be seen that the blank one pixel wide
structure 714 has been "filled" in so that portions 710' and 712
are contiguous with one another. In this manner, printer alignment
problems of one or multiple pixel wide misalignments may be
corrected in this fashion. It is also to be noted that shadow
portion 712 also could have been thickened to correct for the
misalignment issue. For multiple pixel wide alignment problems,
both structures or portions can be thickened to "grow" closer to
one another.
FIG. 20a is an illustration of a binary bitmap of a character 730
in a pixel grid.
FIG. 20b illustrates the one pixel wide outer edge 732 of character
730 identified using the edge and directional capabilities of the
present render algorithm. For example, the pixels of edge 732 would
be identified as either N, NE, or E pixels.
FIG. 20c illustrates how a one pixel wide structure 734 may be
created adjacent or contiguous with structure 730. Structure 734
may have a different density or value than structure 730. In this
manner, a shadow or other effect can be applied to any of a number
of structure in an existing image post RIP.
As discussed earlier, a continuous tone spectrum may be
approximated by turning on a certain percentage of the pixels in
each halftone cell. The algorithm described hereinbefore provides
the ability to independently control the line width/dot gain and
solid area density of a binary ripped image in a printer which uses
a multibit writer. Pixels defined by the ripping process are
classified as background pixel, edge pixel, interior pixel, part of
a 1 pixel wide line pixel, or part of a 2 pixel wide line pixel.
Each pixel classification can be assigned a multibit value. By
appropriate selection of values the dot gain/line width of the
image can be modified as well as the solid area density. It has
been demonstrated that line width and solid area density settings
can be selected so that the line width can be adjusted
independently of solid area density. As seen in FIG. 9, a GUI may
be provided to a user in which the settings can be independent of
each other. One of the sliders can be labeled "linewidth/dot gain"
and another can be labeled "solid area density". At a given solid
area density, the line width slider can be changed to obtain
varying line widths without impacting the solid area density of
text. Conversely, at a given line width setting, the solid area
density slider can be changed to obtain different solid area
densities while maintaining the same line width. However, changing
linewidth/dot gain affects text linewidth as well as halftones and
text/line art.
Therefore, if a ripped image contains both halftones areas and
text/line art, it is possible that the desired linewidth/dot gain
setting does not achieve the desired results for both halftones and
text/line art. As an example, one set of linewidth/dot gain and
solid area density settings yields the desired result for the
halftone image on a page but is not satisfactory for the text/line
art. A different set of settings may be required to attain the
desired results for the text/line art.
The post RIP RENDER algorithm described hereinbefore provides a way
to change text/line art. But these changes impact halftones as well
because the algorithm operates globally (on all pixels in the
image). These changes to halftones, however, might not be desired
by the customer. The present invention provides a means to change
text/line art without changing halftones, or changing text/line art
and halftones independently.
Referring now FIG. 21, a method of changing text/line art and
halftones independently involves a first step 802 to produce a set
of sample tone levels. An example of a possible sample set is shown
in FIG. 12. Other samples, however, may be used. For ease of
illustration, FIG. 12 shows increments of 10% white. Other
increments, such as 10% or 2.5% white may be used. In a step the
804, one or more of the IPV, EPV, 1PV, or 2PV values are reassigned
according to the post rip RENDER circuit described hereinbefore. In
a step 806, a second set of sample tone levels is then produced.
The second set of sample tone levels are selected the same as or
similar to the first set, so that a comparison of the first set and
second set in a step 808 will identify how the changes in step 804
have effected tone levels. The operator of the marking device may
compare the sample tone levels either visually or using
instrumentation, or the comparison may be automatically performed
by, for instance, a scanner. One or more calibration set points are
then determined in step 810. The calibration set points are
determined by identifying a tone level from the first set that most
closely matches a tone level from the second set. For instance, the
40% white tone level on the first sample output might match the 30%
white tone level on the second sample output. In this case, the
density of the 40% white tone level on the first sample output most
closely approximates the density of the 30% white tone level on the
second sample output. The relationship between these two sample
tone levels defines a calibration set point between the print
without post RIP rendering and print after post RIP rendering is
activated. This comparison process can be repeated for more of the
sample tone levels in FIG. 12 to generate more calibration set
points. Between ten and twenty calibration set points is an
exemplary amount to be determined. However, more or less
calibration set points may be used, depending upon the
circumstances.
Once the calibration set points have been determined, they are used
to plot a custom or "UNDO" transfer curve of pre-RIP rendering tone
levels vs. post-RIP rendering tone levels in a step 812. In this
manner, appropriate post-RIP rendering tone level are mapped or
calculated for a requested pre-RIP tone level. For instance, a
calibration set point might indicate that a 30% white tone level
printed from a post RIP rendering process produces the same output
tone density as a 20% white tone level printed with no post RIP
rendering. The result is that tone densities are lighter when post
RIP rendering is utilized. The UNDO transfer function of the
present invention corrects for this.
The UNDO tone transfer curve or function thus generated is
associated with the selected post RIP rendering pixel values (e.g.
IPV, EPV, 1PV, 2PV, etc.) in a step 814. In this manner, an
operator may override the printer standard tone transfer function
(e.g. via an undo button or command from the operator interface)
with the UNDO tone transfer function to UNDO halftone/dot gain
changes that otherwise would have occurred with the post RIP
rendering function selected in a step 804. It is to be noted that
the UNDO transfer function may be embedded into the digital image
file before rasterization.
The generation of an UNDO tone transfer function (step 812) may be
performed according to any of a number of different statistical
methods. Given one or more calibration set points, a function
should be defined that includes each of the set points. FIG. 22
shows a graph of output percentage white produced versus input
percentage white requested for four different post RIP rendering
settings. Typically, marking devices use a unity tone transfer
function 830 to map input to output. At one end of the unity
transfer function, a 0% white tone level request corresponds to a
0% tone level output. Likewise, at the other end, a 100% white tone
level request corresponds to a 100% tone level output. In between
these end points the unity transfer function produces a similar
one-to-one correspondence for all input values. An UNDO tone
transfer function differs in that it incorporates one or more
calibration set points, such as set points 832a-d. For instance,
the calibration set points of FIG. 21 are linked using linear
segments. Alternatively, smooth curve segments may be used.
The method described above has generally been described with
reference to the gray tone levels produced by a black and white
printer. This method is equally applicable to systems that produce
color output. In the case of a color printer that uses red, green,
and blue (rgb) as its component colors, a separate undo tone
transfer function should be generated for each of the three
component colors. Similarly, in the case of a printer that uses
cyan, yellow, magenta, and black as its component colors, a
separate undo tone transfer function should be generated for each
of those four component colors. In each of these cases, first and
second sets of sample tone levels are produced for comparison of
different shades of each component color. Additionally, the method
can be applied to printers that produce accent colors and printers
with any number of specialized supplemental color systems. Again,
first and second sets of sample tone levels would be produced for
each of the accent or supplemental colors.
Referring to FIG. 1a, collectively, the UNDO transfer function,
RIP, MIP, and RENDER circuits may comprise an image processing
subsystem. As a whole, the image processing subsystem receives an
image from the client application, adjusts the tone transfer
function to generate an UNDO tone transfer function, and processes
the image using the UNDO tone transfer function to create a
rasterized pixel stream. The rasterized pixel stream is then used
by the marking engine to produce an output that is in accordance
with the method described above.
The calibration set points may be sent from a calibration interface
to the raster image processor, which generates an undo tone
transfer function in accordance with the method described above.
Alternatively, according to another presently preferred embodiment,
the calibration set points may be sent to the client, and the undo
tone transfer function may be generated and applied at the client
level, as opposed to the marking-device level. In any of these
cases, the undo tone transfer function may take the form of a
lookup table that maps a given output tone level to a desired
output tone level.
The logical operation of a process in accordance to that described
above will now be discussed with reference to FIG. 23. First, a
counter variable "n" is initialized and set equal to one in step
902. Next a sample tone level is selected from a first sample
output in step 904. The sample tone level, for instance, may be
based on its location at a pre-arranged position on the first
sample output page. A variable, which may be referred to as white
%1, is then set equal to the value of the percentage whiteness of
the selected sample tone level in step 906. Next, in step 908, a
sample tone level is selected from the second sample output. A
second variable, which may be referred to as white %2, is then set
equal to the value of the percentage whiteness of the selected
sample tone level in step 910. Then, in step 912, the program
determines whether white %1 is a close match for, or is
approximately equal to, white %2. If there is a close match, the
program goes to a step 915 to interpolate to achieve the best match
and then to a step 918. If, however, there is no match, the program
proceeds to step 914, in which the next sample tone level is
selected from the second sample output. The program then determines
whether the selected sample tone level is the last on the second
sample output. If so, the program proceeds to step 918. If not, the
program returns to steps 910 and 912 to determine whether this
newly selected sample tone level from the second sample output
matches the selected sample tone level from the first sample
output.
In step 918, the program saves a calibration set point for the
values (x.sub.n=white %1, y.sub.n=white %2). This occurs whenever a
match is determined in step 912 or the last sample tone level from
the second sample output has been compared with a given sample tone
level from the first sample output. Once a calibration set point
has been saved in step 918, the program determines whether the
selected sample tone level is the last sample tone level on the
first sample output in a step 920. If so, the program skips to step
926. If not, the program proceeds to step 922 to select the next
sample tone level from the first sample output in step 920. The
counter n is then incremented in step 924, and the program returns
to step 906 to begin the process of determining a calibration set
point for the newly selected sample tone level. In step 926, which
commences once all of the calibration set points have been saved,
the program generates an UNDO tone transfer function in accordance
with the saved calibration set points. This may be accomplished
according to the methods described above.
The basis of this invention is the creation of a transfer function
which is submitted to the RIP. The transfer function is computed
based on two inputs. One is the known effects of the post RIP
algorithm settings which are selected by the user. The other is the
desired halftone results when the job is printed.
As an example, a user may desire a change to the line width on the
text part of his job, yet he is satisfied with the halftone result
he achieves when there is no line width/dot gain adjustment. A
transfer function can be computed to negate the effects the POST
RIP algorithm (which is applied globally to the entire image) has
on the halftones. This transfer function could be stored for each
of the users print settings and employed when the user selects to
maintain halftone appearance on the GUI.
Another example is a situation where the user is unhappy with the
halftone result obtained at the settings which are optimal for
text/line art. The user can then identify changes he wants in the
halftone image. The user may specify desired halftone changes
relative to the selected text/line art settings or relative to the
base line settings (no text/line art adjustments). The method or UI
(user interface) by which the user specifies the desired halftone
results are numerous. The end result of specifying the halftone
print results is a transfer function. Because the computed transfer
function is applied only to halftones, the net result is
independent settings for halftones and text/line art. This is
accomplished for this type of job without the need for complex and
artifact prone image segmentation algorithms.
While the present invention has been described according to its
preferred embodiments, it is of course contemplated that
modifications of, and alternatives to, these embodiments, such
modifications and alternatives obtaining the advantages and
benefits of this invention, will be apparent to those of ordinary
skill in the art having reference to this specification and its
drawings. It is contemplated that such modifications and
alternatives are within the scope of this invention as subsequently
claimed herein.
It should be understood that the programs, processes, methods and
apparatus described herein are not related or limited to any
particular type of computer or network apparatus (hardware or
software), unless indicated otherwise. Various types of general
purpose or specialized computer apparatus may be used with or
perform operations in accordance with the teachings described
herein. While various elements of the preferred embodiments have
been described as being implemented in software, in other
embodiments hardware or firmware implementations may alternatively
be used, and vice-versa.
In view of the wide variety of embodiments to which the principles
of the present invention can be applied, it should be understood
that the illustrated embodiments are exemplary only, and should not
be taken as limiting the scope of the present invention. For
example, the steps of the flow diagrams may be taken in sequences
other than those described, and more, fewer or other elements may
be used in the block diagrams.
The claims should not be read as limited to the described order or
elements unless stated to that effect. In addition, use of the term
"means" in any claim is intended to invoke 35 U.S.C. .sctn.112,
paragraph 6, and any claim without the word "means" is not so
intended. Therefore, all embodiments that come within the scope and
spirit of the following claims and equivalents thereto are claimed
as the invention.
* * * * *